Trump has been the presumptive GOP nominee since May 3, when Sen. Ted Cruz and Ohio Gov. John Kasich dropped out of the race, leaving him the last of the 17 Republicans standing.

But Thursday's milestone solidifies that status, officially killing the possibility of a contested Republican National Convention in Cleveland in July.

On the Democratic side, Hillary Clinton is still competing with Sen. Bernie Sanders. But she is expected to clinch her party's nomination on June 7, when voters in New Jersey and California head to the polls.